age of Arthur's seat, Edinburgh. 141 



folding of the strata. The general tendency of these series of 

 parallel faults, some of which are of great magnitude, has been to 

 bring the highest or newest beds into the lowest positions in the 

 trough of the Central Valley. 



But besides these great series of parallel rolls and faults in the 

 Newer Palaeozoic strata of Central Scotland, they are also traversed 

 by a great number of fissures, which are filled with various igneous 

 products, and everywhere maintain, with the most striking constancy, 

 the same general direction as the faults and foldings of the strata. In 

 1824 Sir Charles Lyell described in detail the interesting appear- 

 ances presented by one of these great fissures, at the Den of the 

 Carity and at other points in the county of Forfar* ; and I have 

 recently enjoyed the privilege of studying under his guidance the 

 structure of the whole of that district, to which during more than 

 fifty years he has devoted so much attention. 



Leaving out of consideration for the present those older volcanic 

 outbursts, the products of which constitute the Ochil, Sidlaw, Pent- 

 land and other similar hill- ranges, we find, in Forfarshire, Fife, and 

 the Lothians respectively, a most instructive series of the products 

 of igneous activity during the Carboniferous period displayed along 

 the great parallel lines of fissure we have referred to, the volcanic 

 structures thus produced being presented to our study in every 

 conceivable stage of unfolding by denudation. 



I have spoken of these volcanic outbursts as being of Carboni- 

 ferous age. The remarkable parallelism of the lines along which 

 they are manifested, taken in connexion with the striking similarity 

 of their products, leaves little, if any, room for doubt that they all 

 belong to the same geological period. What that period was, the 

 lavas and ash-beds of southern Fife and the Lothians, so unmista- 

 kably seen to be interbedded with the Carboniferous strata, clearly 

 testify. 



Of series of volcanic cones evidently arranged along a number of 

 parallel lines, which doubtless indicate the existence of subterranean 

 fissures similar to those of Central Scotland, many examples might 

 be cited among still active volcanic districts. Nowhere, perhaps, is 

 this arrangement better illustrated, however, than in the familiar 

 Campi Phlegrsei of Naples. 



In the more northern of the districts which we have referred to 

 as exhibiting proofs of these outbursts, that of Forfar, the maximum 

 effects of denudation have been experienced, and we find the various 

 rocks of the Old-Red- Sandstone period traversed by numerous fissures, 

 varying in width from a few feet to several hundred yards ; and 

 these, in spite of the drifts, which so greatly obscure the country, 

 can be frequently traced for many miles. The same fissure often 

 varies greatly in width at different points ; but sometimes they are 

 found, like ordinary dykes, maintaining a parallelism of walls and 

 constancy of breadth for long distances. These fissures are all 

 filled with the products of volcanic action, either solidified lavas or 



* "Oua dyke of serpentine cutting through sandstone in the county of Forfar," 

 Edinb. Journ. Sci. iii. (1825), pp. 112-126. 



